This invention relates to a topical capsaicin preparation. Capsaicin, the primary pungent principle in the fruit of capsicum plants, produces marked alterations in the function of a defined subpopulation of unmyelinated sensory afferents, termed C-polimodal nociceptors. Following the initial period of intense burning or stinging pain accompanied by erythema, topical capsaicin application causes insensitivity to further irritation by a variety of noxious stimuli. Accordingly, topical preparations of capsaicin find use as a topical therapy for a variety of cutaneous disorders that involve pain and itching, such as postherpetic neuralgia, diabetic neuropathy, pruritus, psoriasis, cluster headache, postmastectomy pain syndrome, rhinopathy, oral mucositis, cutaneous allergy, detrusor hyperreflexia, loin pain/hematuria syndrome, neck pain, amputation stump pain, reflex sympathetic dystrophy, pain due to skin tumor and arthritis. Martin Hautkappe et al., Review of the Effectiveness of Capsaicin for Painful Cutaneous Disorders and Neural Dysfunction, Clin. J. Pain, 14:97-106, 1998.
Because of intense burning or stinging pain, many patients are not tolerated in the long-term treatment with topical capsaicin and, therefore, have to discontinue the treatment before appearance of analgesic effect of capsaicin through prolonged administration. It was reported that 26 out of 39 (66.7%) patients suffering from postherpetic neuralgia were not tolerated with a 0.025% capsaicin preparation (Zostrix, Gen Derm, USA). With a 0.075% preparation (Zostrix-HP, Gen Derm, USA), 5 out of 16 (31.3 %) and 45 out of 74 (60.8%) patients with postherpetic neuralgia were not tolerated. Peikert, A. et al., Topical 0.025% capsaicin in chronic post-herpetic neuralgia: efficacy,predictors of response and long-term course, J. Neurol. 238:452-456, 1991; Watanabe, A. et al., Efficacy of capsaicin ointment (Zostrix) in the treatment of herpetic pain and postherpetic neuralgia, Pain Clinic 15:709-713, 1994; Bernstein J. E. et al., Topical capsaicin treatment of chronic postherpetic neuralgia, J. Am. Acad. Dermatol. 21: 265-270, 1989; and Watson C. P. N. et al., A randomized vehicle-controlled trial of topical capsaicin in the treatment of postherpetic neuralgia, Clin. Ther. 15:510-526, 1993.
A need exists for a topical capsaicin preparation which eliminates or substantially ameliorates initial stinging pain caused by capsaicin observed in the administration thereby making the preparation tolerable in long-term administration.
We have found that the initial stinging pain caused by capsaicin is eliminated or substantially ameliorated by incorporating a large excess of nonionic, amphoteric or cationic surfactants into the topical capsaicin preparation.
Accordingly, the present invention provide a topical preparation comprising (a) an amount of capsaicin effective in long-term or repeated administration to cause insensitivity to painful stimuli associated with painful cutaneous disorders and neural dysfunction, and (b) an amount of a nonionic, amphoteric or cationic surfactant effective to eliminate or substantially ameliorate the initial stinging pain caused by capsaicin, in admixture with a pharmaceutically acceptable carrier for topical administration. Preferably, capsaicin is the sole agent acting on the nervous system contained in the preparation.
The term xe2x80x9csubstantially amelioratexe2x80x9d as used herein refers to at least 50%, preferably at least 70% and most preferably at least 90% of patients can tolerate the long-term administration of capsaicin to cause insensitivity to painful stimuli. The amount of surfactants to achieve the above effect lies in the range between about 5% and about 20% and preferably in the range between about 9% and 18% by weight of the preparation. When combined with the surfactant, the amount of capsaicin in the topical preparation may be increased to at least about 0.1% by weight although 0.075% of capsaicin has been believed to be maximum.
As is known, capsaicin acts on C fibers which serve to transmit the pain impulse to the central nervous system. Initial administration of capsaicin stimulates the C fibers to cause intense burning or stinging pain. Continued administration thereof, however, suppresses the inherent function of these sensory nerve fibers to cause insensitivity to painful and other sensory stimuli.
As a preliminary study, we have tested certain surfactants for the effect on the neuropharmacology of capsaicin.
1. 0.1% capsaicin solutions containing varying amount of polyoxyethylene (60) hydrogenated castor oil were prepared by dissolving 0.1 g of capsaicin and 0, 3, 9 or 18 g of the surfactant in 26.4 ml of ethanol and then diluting with distilled water q.s. to make a total volume of 100 ml.
Each of 0.1% capsaicin solution thus prepared was applied to the hind-limb of rats. One hour after the application, the limb was placed in a water bath warmed at 42xc2x0 C. and the length of time (in seconds) elapsed until when the rat withdrew the treated limb from the water bath was determined. The results are shown in Table 1.
As shown in Table 1, application of 0.1% capsaicin solution without polyoxyethylene hydrogenated castor oil remarkably shortened the withdrawal latency compared to the withdrawal latency measured before application of the test solution. The withdrawal latency was slightly shortened at a surfactant concentration of 3% but was not affected significantly at a surfactant concentration of 9% and 18%, respectively.
These results demonstrate that the response of sensory nerves to thermal stimuli during sensitization by capsaicin is suppressed by co-administration of polyoxyethylene hydrogenated castor oil at a concentration of 9% or higher.
2. The effect of polyoxyethylene (60) hydrogenated castor oil on the eye-wiping reflex caused by capsaicin and the desensitization to chemical stimuli was studied according to the eye-wiping method reported by Jancso N. et al., in xe2x80x9cDirect evidence for neurogenic inflammation and its prevention by denervation and by pretreatment with capsaicinxe2x80x9d, Br. J. Phamac. Chemother. 31: 138-151, 1967.
When one drop of 0.1% capsaicin solution was applied to the cornea of rats, the animal immediately began to wipe the cornea with front paws and continued the wiping for about 1 minute. The number of this wiping (first wiping) was not virtually affected by the addition of polyoxyethylene hydrogenated castor oil at a concentration of 3%, 9% or 18% to the 0.1% capsaicin solution.
The pretreatment with 0.1% capsaicin solution containing the surfactant at 0%, 3%, 9% or 18% significantly reduced the number of wiping (second wiping) caused by 0.1% capsaicin solution without the surfactant applied 2 hours after the pretreatment. See, Table 2. The pretreatment with the solvent containing the surfactant alone at 0%, 3%, 9% or 18% did not reduce the number of wiping caused by 0.1% capsaicin solution applied 2 hours after the pretreatment. See, Table 3.
The above test results demonstrate that polyoxyethylenehydrogenated castor oil does not affect the desensitization of sensory nerves caused by capsaicin.
3. The effect of various surfactants on the neural stimuli caused by capsaicin was evaluated by repeating the procedure of Test 1 using test solutions containing capsaicin at 0.1% and a surfactant at 9% or 18%.
The test solutions were prepared by dissolving 0.1 g of capsaicin and 9 g or 18 g of a selected surfactant in 26.4 ml of ethanol and then diluting with distilled water q.s. to make the total volume of 100 ml. The control solution was identical to the test solutions except for the exclusion of the surfactant.
The surfactants used are shown in Table 4 below.
As in Test 1, the length of time elapsed until when the rat withdrew the paw from the water bath warmed at 42xc2x0 C. was determined before and 1 hour after the application of the test solution. At a surfactant concentration of 9%, the withdrawal latency was not virtually shortened one hour after the application of the sample solution while the control solution without surfactant largely shortened the withdrawal latency. See, Table 5. At a surfactant concentration of 18%, all sample solutions except solution No. 8 containing sodium polyoxyethylene (2) lauryl ether sulfate did not shorten the withdrawal latency. See, Table 6.
The above test results demonstrate that the response of sensory nerves to thermal stimuli during sensitization by capsaicin is significantly suppressed by co-administration of various surfactants at a concentration of 9% or higher.
4. The effect of various surfactants on the desensitization of sensory nerves by 0.1% capsaicin was evaluated using the procedure of Test 2.
The sample solution contained capsaicin at 0.1% and a surfactant listed in Table 4 at 18%.
The pretreatment with sample solutions significantly reduced the number of wiping (second wiping) caused by 0.1% capsaicin solution without the surfactant applied one hour after the pretreatment. See, Table 7.
The pretreatment with solutions only containing the surfactants did not affect the number of wiping (second wiping) caused by 0.1% capsaicin solution without the surfactant applied one hour after the pretreatment. See, Table 8.
The above test results demonstrate that the addition of various surfactants to 0.1% capsaicin does not affect the known desensitization of sensory nerves with capsaicin itself.
The topical capsaicin preparation of this invention finds use in the treatment of painful cutaneous disorders and neural dysfunction including but not limited to postherpetic neuralgia, diabetic neuralgia, pruritus, psoriasis, cluster headache, postmastectomy pain syndrome, rhinopathy, oral mucositis, cutaneous allergy, detrusor hyperreflexia, loin pain/hematuria syndrome, neck pain, amputation stump pain, reflex sympathetic dystrophy, pain due to skin tumor and arthritis.
The topical capsaicin preparation of this invention may take the form of liquids, ointments, creams, gels, plasters or other forms adapted for topical application. These preparations may be manufactured by the methods well-known in the art and may comprise a mineral oil such as liquid paraffin or vaseline, a fatty alcohol such as cetyl or stearyl alcohol, a gelling agent such as carboxyvinyl polymers or fatty amines, and other conventional additives such as preservatives, perfumes, coloring agents and the like. Capsaicin is hardly soluble in water but easily soluble in oils and ethanol. Because of this, the topical capsaicin preparation preferably comprises a medium in which capsaicin is easily soluble.
A variety of nonionic, amphoteric and cationic surfactants are known and may be used in the present invention. Non-limiting examples of nonionic surfactants included polyoxyethylene castor oil such as polyoxyethylene (10) castor oil; polyoxyethylene hydrogenated castor oil such as polyoxyethylene (60) hydrogenated castor oil; polyethylene glycol fatty acid ester such as polyethylene glycol (45) monostearate; polyoxyethylene alkyl ether such as polyoxyethylene (20) oleyl ether; polyoxyethylene-polyoxypropylene alkyl ether such as poloxamer 235; polyoxyethylene alkylphenyl ether such as polyoxyethylene (7.5) nonylphenyl ether; and polyoxyethylenesorbitol fatty acid ester such as polyoxyethylene (60) sorbitol tetraoleate. Examples of amphoteric surfactants include betaine derivatives such as N-dodecyl-N,N-dimethylglycine betaine. Example of cationic surfactants include cetyltrimethylammonium chloride.
Now the present invention will be described with reference to the following Examples.